1. Field of the Invention
The present invention relates to polymeric compositions, particularly partially neutralized acid copolymers with enhanced melt swell and, thus, improved processing characteristics. It also relates to a process for covalently crosslinking polar and non-polar alpha olefin polymers employing metal acrylates.
2. Description of Related Art
Ionomers (metal salts of acid copolymers) are prepared by methods well known in the art (see U.S. Pat. No. 3,264,272 (Rees) which is hereby incorporated by reference). Likewise, acid copolymers on which the ionomers are based are prepared by methods well known in the art (see U.S. Pat. No. 4,351,931, which is also incorporated by reference).
The acid copolymers typically used are direct acid copolymers. By “direct copolymer”, it is meant that the copolymer is made by polymerization of monomers together at the same time, as distinct from a “graft copolymer” where a monomer is attached or polymerized onto an existing polymer chain. They typically are alpha olefin, particularly ethylene/C3-8 α,β ethylenically unsaturated carboxylic acid, particularly acrylic and methacrylic acid, copolymers. They may also contain a third softening monomer such as an alkyl acrylate or methacrylate. The ethylene acid copolymers can be described as an E/X/Y copolymers where E is ethylene, X is the α,β ethylenically unsaturated carboxylic acid, particularly acrylic and methacrylic acid, and Y is a softening comonomer such as acrylate or methacrylate ester.
Ionomers are produced by reacting an ion source with the acid copolymers. Typical ion sources include sodium hydroxide, sodium carbonate, zinc oxide, zinc acetate, magnesium hydroxide, and lithium hydroxide. Other ion sources are well known. In addition to the preferred sodium, zinc, magnesium and lithium cations, other alkali metal or alkaline earth metal cations are useful. Such other cations include potassium, calcium, barium, lead, and tin.
This ionic crosslinking produces a thermoplastic polymer that does not exhibit appreciable increase in the elastic component of flow and, therefore, produces a thermoplastic resin with low melt swell. Processing characteristics are thus adversely effected. By increasing melt swell, ionomers with improved Theological characteristics at a given viscosity useful in extrusion coating, blown film and molding applications could be made.